This invention relates to a sintered alloy which exhibits superb wear resistance at elevated temperatures and a process for producing such a sintered alloy. Particularly, the invention relates to the technique suited to be used for a valve seat of internal combustion engines.
Recently, with the progress of higher performance of automobile engines, the conditions for operation became much harder. The valve seat used for such engines is also inevitably required to withstand severer environments than ever. To fulfill such requirements, the present applicant previously proposed several sintered alloys having superb wear resistance as disclosed, for example, in Japanese Examined Patent Publication (KOKOKU) Nos. 17968/74, 36242/80, 56547/82, 55593/93, and 98985/95.
Of all the proposed sintered alloys having superb wear resistance, the alloy disclosed in the Japanese Patent Examined Publication (KOKOKU) No. 56547/82 is improved in the wear resistance. The sintered alloy having such improved wear resistance is produced by preparing, in order to form a matrix for a valve seat, two kinds of alloy powders by alloying all the components contained in the alloys disclosed in the Japanese Patent Examined Publication (KOKOKU) Nos. 17968/74 and 36242/80 only excepting the graphite, then mixing a graphite powder to the alloy powders thus prepared, and then forming and sintering the mixed powder so that a porphyritic matrix structure will be exhibited. Specifically, graphite is added to a mixed powder of A alloy powder consisting, percent by weight, of Cr in the amount of 2 to 4%, Mo in the amount of 0.2 to 0.4%, V in the amount of 0.2 to 0.4%, and the balance of Fe, and B alloy powder consisting, percent by weight, of Ni in the amount of 0.5 to 3%, Mo in the amount of 0.5 to 3%, Co in the amount of 5.5 to 7.5%, and the balance of Fe to set the weight ratio between the A alloy powder and the B alloy powder to 25:75 to 75:25. By doing so, the wear resistance is improved.
The sintered alloy having superb wear resistance disclosed in the Japanese Patent Examined Publication (KOKOKU) No. 55593/93 exhibits a structure obtained by dispersing a hard phase consisting, percent by weight, of Mo in the amount of 26 to 30%, Cr in the amount of 7 to 9%, Si in the amount of 1.5 to 2.5% and the balance of Co to the alloy disclosed in the Japanese Patent Examined Publication (KOKOKU) No. 36242/80. The sintered alloy having such a structure can be produced by compacting and sintering a mixed power of the B alloy powder, C alloy powder consisting, percent by weight, of Mo in the amount of 26 to 30%, Cr in the amount of 7 to 9%, Si in the amount of 1.5 to 2.5%, and the balance of Co, and a graphite powder. The sintered alloy having superb wear resistance disclosed in the Japanese Patent Examined Publication (KOKOKU) No. 98985/95 is an improvement of the alloy disclosed in the Japanese Patent Examined Publication (KOKOKU) No. 55593/93. By containing the alloy of the Publication (KOKOKU) No. 55593/93 with Ni in the amount of 5 to 27% by weight, the matrix structure is strengthened.
As apparent from the foregoing, the present applicant proposed several sintered alloys having superb wear resistance in order to meet with the requirements of the age. However, the present situation is that the conditions for operation are even increased in severeness as the performance of automobile engines is further improved and that the material, which is superior in wear resistance and in strength at elevated temperatures to the aforementioned sintered alloys, is demanded.
For instance, in the LPG fuel engine loaded on taxicabs, the contact surfaces between the valve and the valve seat are dry in use. For this reason, the valve seat of the LPG fuel engine is worn more quickly than the valve seat of the gasoline engine. In the environments where sludge is liable to attach as in the lead-contained gasoline engine, in case the surface pressure with respect to the valve seat is high, or in case of a high temperature/high compression ratio as in the diesel engine, the wear is further promoted by sludge. For the use in such hard environments, in addition to the superb wear resistance, a physical strength large enough to prevent permanent set in fatigue is demanded.
On the other hand, a valve gear mechanism having a rush adjuster mechanism capable of automatically adjusting the position of the valve and the timing for driving the valve even in a worn condition of the valve seat, is already put into actual practice. However, the problem associated with the service life of the engine due to wear of the valve seat is not yet obviated. Also in view of the strong demand for lowering the cost, the development of a valve seat material having superb wear resistance is demanded.
The development of automobiles of the recent year is directed not only to higher performance but also to lower cost from an economical view point. For this reason, the sintered alloy for such valve seats are required to have superb wear resistance at elevated temperatures and large physical strength without a need of additional mechanisms mentioned above.
The present invention has been accomplished in view of the above-mentioned situation.